It is already well-known that recent steel sheets for vehicles are gradually increased in strength due to a demand for reduction in fuel cost as well as higher safety of a passenger in the event of collision, are required to have a higher level of formability due to a tendency toward complication and integration of vehicle parts, and are required to have excellent secondary working brittleness resistance, superior fatigue characteristics of a weld zone, and beautiful plated surface in the terms of the environments in which the vehicles are used. As well-known up to now, in order to increase formability and strength, the steel sheets are generally manufactured by adding structure reinforcement elements such as C, Si, Mn, Ti, Al and so on. These elements function to form a metastable transformation structure during quenching, martensite or bainite or austenite retained to room temperature (hereinafter, referred to as “retained austenite”) without transforming austenite formed at high temperature into ferrite and cementite, or pearlite at room temperature, thereby obtaining appropriate strength and ductility.
According to the disclosures of Japanese Patent Publication Nos. 2005-187837 and 2004-346362, C, Si and Mn are main components, and either a solution strengthening element, P, causing press formability for strength to be less reduced or Al having characteristics similar to Si is added. Contents of Si and Al are limited, and B or various components such as rare earth metals are added in order to improve working brittleness. However, the components other than the main components have an obscure effect, and description of some of the elements is far apart from typical metallurgical knowledge. For example, in the case of B, since C contained in high strength steel at a great amount can sufficiently prevent grain boundary embrittlement, a quench hardening effect is further increased due to B. As a result, B shows a tendency to deteriorate the working brittleness.
Further, according to the disclosure of Japanese Patent Publication No. 2000-368317, restrictions are intentionally imposed on composition and production conditions in order to improve press formability with a composition nearly similar to the aforementioned known technologies. This also has a little effect. In fact, in the continuous casting-hot rolling process, these elements degrade high-temperature ductility to weaken steel at high temperature, and cause surface enrichment during cold-rolled annealing because they have higher oxygen affinity in comparison with Fe. Thus, these elements generate bare spots, and thus readily deteriorate plating quality. Furthermore, when the surface enrichment is coarsened, it is adsorbed to the hearth roll of a continuous annealing line, and thus is apt to cause micro-dents in the surface of a plated steel sheet.
In order to cope with the plating defects as described above, technology for manufacturing a high strength thin steel sheet for high press formability is disclosed in Japanese Patent Publication Nos. 2002-146477, 2001-64750, 2002-294397, 2002-155317 and 2001-288550. Describing the disclosures in brief, specific elements such as Cr, Sb, Sn, etc. are added to improve platability, or a hot-rolled coil is previously oxidized prior to cold rolling, thereby inhibiting the surface enrichment formed during cold rolling annealing. However, these disclosures fail to give a positive effect of adding the specific elements or a definite study on metallurgical behaviors of the added elements, and thus do not give a complete manufacturing method required to obtain the effect. Furthermore, some of the disclosures are directed to the manufacturing method that cannot be implemented using current typical hot rolling-cold rolling-continuous annealing equipment, so that they do not apply to actual commercial production.